Method and apparatus for transformer tap changing under load



March 22, 1938. L, BLUME 2,112,064

METHOD AND APPARATUS FOR TRANSFORMER TAP CHANGING UNDER LOAD Filed Jan. 21, 1937 ADJUSTER Inventor: Lcuis F. Blurne,

His A'tt Orneg.

Patented Mar. 22 1938 METHOD AND APPARATUS FOR TRANS- FORMER TAP CHANGING UNDER Low Louis F. Blume, Plttsfleld, Mass., assignor to General Electric Company, a corporation ot New York Application January 21, 1937, Serial No. 121,619

10 Claims;

My invention relates to transformer tap changing and more particularly to a new and improved method and apparatus for transformer tap changing under load in heavy duty circuits.

The standard way of changing taps under load on large power transformers is as follows: The transformer current is initially divided between two branch conductors which are each connected to the same tap on one of the transformer wind ings. A switch then interrupts the current in one branch conductor. A tap change is then made by the transformer ratio adjuster with respect to that conductor. During this time all the transformer current flows through the second branch conductor. The circuit through the first conductor is then reclosed, resulting in a redivision of the transformer current between the two branch conductors. Then a second switch interrupts the current in the'second branch conductor and the ratio adjuster makes another tap change with respect to the second branch conductor. The circuit through the second branch conductor is then reclosed. Inthis way taps are changed in a step by step manner along a row of transformer tap contacts without at any time breaking the main circuit.

Heretofore, such arrangements have required the use of at least two current interrupting arcing contactors or switches, one being connected in each of the two branch circuits or conductors. These contactors have to be capable of carrying the full circuit current continuously so that they are relatively large and expensive and they require a relatively high contact pressure between their contacts.

In accordance with my invention, I separate the two functions of (i) selecting which of the two branch conductor circuits is to be opened and (2) interrupting the current in the selected branch. In doing this, I use one or more main current carrying switches for selecting which of the two branch circuits is to be opened, and I utilize a separate switch or contacto-r for performing the current interrupting function.

The selector switch or switches need not be designedfor arcing duty and they may consist of little more than low resistance separable contacts which are capable of continuously carrying the transformer current. The current interrupt ing switch need only be an arcing element and need not be designed to carry the full circuit current continuously. It is therefore possible to construct it of light material and for relatively low contact pressure.

55 It is to be noted that I utilize but a single current interrupting contactor instead of two as heretofore.

An object of my invention is to provide a new and improved method and apparatus for transformer cap changing under load.

Another object of my'invention is to provide a new and improved way of heavy duty transformer tap changing under load.

My invention will be better understood from the following description taken in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

.Referring now to the single figure of the accompanying drawing, have shown therein diagrammatically an embodiment of my invention as applied to a power transformer I. As shown, this transformer has a primary winding 2 connected to a supply circuit 3 and a secondary winding 4 connected to a load circuit 5. The winding 4 is provided a plurality of tap contacts ii. 1 suitable ratio adjuster oi any well known construction is arranged to make selective connections between tap contacts 6 and a pair of branch circuit conductors and Q. The mechanical details of one such ratio adjuster are shown in. Patent No. 1,588,204, issued June 8, 1936 on an application of H. O. Stephens et a1. and assigned to the assignee of the present application. The branch conductors i3 and 9 are arranged for selective simultaneous or individual connection to the lowermost conductor of load circuit by means or" a main current carrying selector switch it. This switch is really a non-current interrupting, disconnecting switch and is shown by way of example as consisting of a segmental cop- 35 per disc ti which in the position shown bridges a pair of fixed contacts 52 and I3 connected respectiveiy to the branch conductors 8 and 9. A, brush contact 55 makes continuous engagement and electrical contact with the disc H. Due to the cut-away portion in the disc it, rotation through a relatively small angle in either direction causes one or the other of the branch conductors or 9 to be disconnected from the circuit although it will be seen that at no time can both branch conductors be simultaneously disconnected from the main load circuit.

it will be observed that switch it] is the equivalent of the separate normally closed switches which are so interlocked that only one can he opened at a time. Obviously, therefore, switch it may be replaced by such an arrangement.

For interrupting the current in the branch conductors 8 and El 1 provide a single arcing ccntactor or switch it. This switch may be oi" iii) ' erating means. As shown, they are all coupled together by a shaft I9 driven by a motor 20.

' The inter-relation between the ratio adjuster I, the disconnecting switch III and the current interrupting switch I5 is such that as the shaft I! is turned in either direction, first the switch Il breaks the connection through itbetween one or the other of the fixed contacts I2 or I2 and the common contact I4, next the current interrupting switch It opens, and thereafter the ratio adjuster operates to change the tap connection of the disconnected branch circuit conductor. As the rotation of the shaft I8 is continued, the switch -,I5' recloses first, and shortly thereafter the main current carrying switch It closes.

Connected in series with the branch conductors 8 and 9 are the halves 2I of a preventive reactor. Such devices are well-known and serve to limit the circulating current through the branch conductors at times when they are connected to different voltage tap contacts. When the branch conductors, are connected to the same tap contact,'the currents in the two halves 2| of the preventive reactor neutralize each other electromagnetically and no current limiting reactance is set up in the circuit.

By way of example, I have shown my invention as applied to an automatic voltage regulating system. The main control element is a conventional contact making voltmeter 22 connected to respond to the voltage of load circuit 5. It has a set of raise" contacts 23 which close when the load circuit voltage is below a predetermined .normal value and a set of lower contacts 24 which close when the circuit voltage is above the predetermined normal value. These raise" and lower" contacts 23 and 24 control reversing circuits for the motor 28, which is shown by way of example as a conventional reversible capacitor motor, such as is commonly used for driving the tap changing mechanism of regulating transformers. Ordinarily, of course, auxiliary relays are interposed between the contact making voltmeter contacts and the motor circuits, but for the-sake of simplicity, they have been omitted from the drawing.

The operation of the illustrated embodiment of my invention is as follows:

Assume that power is being transmitted through the transformer I from the supply circuit 3 to the loadcircuit 5 and matter some reason the voltage of the load circuit 5 falls sufficiently below normal to cause the engagement of the raise" contacts 23. This completes a circuit for operating the motor 20. Assume that the motor 28 rotates in such a direction that copper disc II, and. cam I8 turn in a counterclockwise direction as viewed in the drawing.

Before anything happens as a result of the operation oi the motor and of the rotation of the disc II and the cam I8, all of the transformer load current will be divided between the branch conductors 8 and 9, which as shown are both connected to the same tap contact by the ratio adjuster I. Practically all of this current will be flowing through switch I0, which is designed to carry all of this current and which has a lower resistance than the current inte rupting contactorl5.

Now, as the shaft It starts to rotate in a counter-clockwise direction, the first thing that happens is that the disc II breaks contact with fixed contact II. However, the current in branch circuit 8 is not interrupted but continues to flow through the contacts of the current interrupting switch I5 around to the fixed contact I2 "and then through the disc II and the common contact I4 to the circuit 5. The fixed contact I2 is thus carrying the entire transformer load current. The cam .II is. so constructed that shortly after the disc II leaves the fixed contact I8 the current interrupting switch I5 is opened thereby interrupting the current fiow through the branch conductor 9. All of the transformer current is now being-carried by the branch conductor 8. The branch conductor 8 can then be connected to the next lowermost tap contact, as viewed in the drawing, either manually or by means of the ratio adjuster I. Shortly after conductor 9 is connected to the next lowermost tap contact, the switch I5 closes thereby completing a. current carrying path from the conductor 9 through the fixed contact I2 and the disc II to the common contact I4. Soon after the closure of the switch I5, the disc II will have rotated substantially 180 and will again form a bridging connection between the fixed contacts I2 and I3. The voltage of the load circuit will now have been raised by an amount corresponding to half the voltage between the tap contacts and if' is is sufiicient to restore the voltage to normal, the contactmaking voltmeter will break the motor circuit through its "raise" contacts 23. If not, the motor 20 will continue rotating and disc I I will soon thereafter break contact with fixed contact I2, but the current through the branch conductor 8 will continue to fiow through the switch I5 to the fixed contact I3 and then through the switch II) to the main circuit. Shortly afterwards, however, the switch I5 will again open thereby interrupting the current in the branch 8 whereupon the branch 8 can be connected by means of the ratio adjuster, or manually, to the same tap contact to which conductor 9 has been conneeted. The switches I5 and III will then close in the order mentioned and the voltage of the load circuit will be raised by one-half the difbranch conductor which is not carrying current being connected to the next lowermost tap, and

then the switches l5 and I6 closing in the order given and so on until the voltage has been raised to the value at which the contact-making voltmeter 22 balances.

If the voltage of the load circuit 5 is too high, the lower contacts of the contact-making voltmeter 24 close thereby reversing the direction of the motor 28.- This does not reverse the sequence of operation of the switches III and I5 but merely reverses the direction of tap changing produced by the ratio adjuster. Thus, instead of connecting the. branch conductors 8 and 9 successively to lower or outer tap contacts, the reversal of direction of the motor 20 causes the ratio adjuster to make successive connections in a. step by step manner of the branch conductors 8 and 9 along the. upper or inner tap contacts, thus lowering the voltage of circuit 5. It is to be understoodQhowever, that before each tap change with respect to one of the branch conductors, the circuitthrough it is first broken by the switch It and then the current in it is interrupted by the switch I 5 before the tap change is made. This operation continues until the voltage is restored to normal whereupon the contact making voltmeter 22 balances.

While I have shown and described a particular embodiment of my invention, it will be obvious to those skilled in the art that changes can be made without departing from my invention, and therefore I aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of transformer tap changing under load which consists of connecting two branch conductors of a circuit to one or more taps on a transformer winding, opening a connection between one of said conductors and said circuit while maintaining a connection between said conductors, opening said connection between said conductors, connecting said disconnected conductor to another tap on said winding, reconnecting said conductors together, and reconnecting the disconnected conductor directly to said circuit.

2. The method of transformer tap changing under load which consists of connecting two conductors that connect to taps on a transformer winding to one side of a circuit through a switch, opening the connection between one of said conductors and said circuit through said switch while maintaining a conducting path for current between said conductor and said circuit through a second switch, opening said path by means of said second switch, connecting .said conductor to another tap on said transformer, closing said second switch and closing the connection between said. conductor and said circuit through the first switch.

3. In, a circuit for transformer tap changing under load, a power circuit, a transformer winding provided with a plurality of taps, a ratio adjuster for selectively connecting a pair of conductors to said taps, main current carrying selector switching means for connecting both of said conductors to said power circuit and for selectively disconnecting either one of said conductors from said circuit, and a current interrupting arcing contactor for maintaining a current conducting path between said circuit and the conductor disconnected therefrom by said switching means until after said switch opens.

4. In a circuit for transformer tap changing under load, a transformer winding provided with.

5. In a transformer tap changing under load circuit, a transformer winding provided with a' plurality of taps, a ratio adjuster for selectively connecting a pair of conductors with a plurality of said tape, a switclrhaving two fixed contacts and a movable contact, said fixed contacts being connected respectively to said conductors, said movable contact being arranged selectively to engage both or either of said fixed contacts, and a contactor connected between said fixed contacts, said switch and contactorbeing so correlated that the contactor is only open when the movable contact of the switch is engaging but one of the fixed contacts.

6. In a circuit for transformer tap changing under load, a power circuit, a transformer winding provided with taps, a ratio adjuster for selectively connecting a pair of conductors to various taps on said winding, fixed contacts connected respectively to said conductors, a movable switch member connected to said circuit and adapted selectively to bridge said fixed contacts or to engage either one of said fixed contacts alone, a contactor connected between said fixed contacts, and means for driving said ratio adjuster and movable member and contactor in such sequence that after the movable member disengages one Q of said fixed contacts, the contactor opens, then said ratio adjuster connects the conductor whose fixed contact has been disengaged to another tap, then the contactor closes and finally the movable member is moved to a position bridging said fixed contacts.

7. In an automatic regulating system, a power circuit, a transformer provided with a ratio adjus'ter for connecting a pair of conductors to taps on a winding thereof, a non-current interrupting disconnecting switch for selectively connecting either one or both of said conductors to said circuit, a current interrupting switch for interrupting the current in whichever conductor has been disconnected from said circuit by said disconnecting switch, and means responsive to an operating condition which is controlled by the ratio of said transformer for driving said ratio adjuster and said two switches in such a manner that the ratio adjuster only operates when the current interrupting switch is open and the current interrupting switch is only open when the disconnecting switch is open. c

8. In combination, a source of current provided with taps, an electric circuit, two conductors, means for connecting said two conductors selectively to said taps, means for connecting said two conductors selectively to said circuit, and a switch connected to said two conductors and completing when in closed position a short circuited loop with said last mentioned means.

9. A system for transformer tap changing under load comprising, a set of fixed contacts, two movable contacts, means to connect selectively said two movable contacts to said" fixed contacts, a load circuit, means to connect selectively said two movable contacts to said load circuit, a single switch arranged to transfer when opened the current from either one of said movable contacts to the other, the direction of transfer being predetermined by a condition of said last means.

\ 10. In combination, a transformer winding with-taps, two movable contacts arranged to engage said taps selectively, an impedance in series with at least one of said movable contacts, a switch to connect said movable contacts selectively to a load circuit, and a switch for bridging "across the conductors of said two contacts at a point intermediate to said impedance and their connection to said load circuit.

toms F, 131mm. 

